1. Field of the Invention
The present invention relates to an aromatic polyimide film directly laminated on one side or on both sides with a metal foil, and processes for preparing the aromatic polyimide film directly laminated with a metal foil.
2. Description of Prior Art
Heat-resistant plastic films (e.g., aromatic polyimide films) laminated with a metal foil are utilized, for instance, for the manufacture of flexible circuit printed boards. Most of known aromatic polyimide films laminated with a metal foil (i.e., composite film) are prepared by combining an aromatic polyimide film and a metal foil using a thermosetting adhesive. The thermosetting adhesive used for combining an aromatic polyimide film and a metal foil is not satisfactorily heat resistant such that the adhesive is able to hold its adhesiveness only up to 200.degree. C. Therefore, most of the known adhesives cannot be employed for the preparation of composite films which are to be subjected to high temperature heat treatment such as soldering or used under high temperatures conditions.
In view of the above problem, several studies have been performed for investigating adhesives having sufficiently high heat resistance. However, heretofore reported adhesives are not practically employable, because some of which require high temperatures for their uses in combining the materials and others require very complicated procedures in their uses. Moreover, in most cases, the composite films obtained by the use of these adhesives hardly show enough adhesiveness in their uses.
In other approaches, composite films having no adhesive layer, that is, aromatic polyimide films directly laminated with a metal foil without use of an adhesive have been studied. For instance, the preparation of a composite film by casting a solution of an aromatic polyimide precursor (i.e., aromatic polyamic acid) on a metal foil and processing the cast layer to form an aromatic polyimide film layer on the metal foil is known. Also known is a process in which an aromatic polyimide film is plated or vapor-deposited with a metal on its surface. The former process utilizing the solution casting has such drawbacks that it is extremely difficult to produce the aromatic polyimide film layer having enough thickness and that a very long time of period is required for removing the solvent or volatile products produced from the film in the course of the film preparation. The process utilizing metal-plating or vapor-deposition also has drawback such that the formation of a metal layer having enough thickness is very difficult.
Japanese Patent Provisional Publication 62(1987)-53827 describes lamination of a metal foil on a thermoplastic polyimide film to prepare a composite film.
Japanese Patent Provisional Publication 62(1987)-104840 and Japanese Patent Publication 61(1986)-15825 describe the preparation of composite films by thermally combining under pressure a metal foil, an aromatic polyimide film having a second-order transition point (Tg) and a highly heat resistant aromatic polyimide film.
However, the composite film prepared using the above mentioned thermoplastic polyimide or the polyimide having Tg shows unsatisfactory heat resistance or thermal dimensional stability in its polyimide film or shows low adhesiveness when it is subjected to high temperature treatments such as treatment in solder bath.
Japanese Patent Provisional Publication Hei-2(1990)-373 describes an aromatic polyimide having high heat resistance and high dimensional stability which is prepared from a biphenyltetracarboxylic acid or its derivative and a phenylenediamine. However, even if the aromatic polyimide having high heat resistance and high dimensional stability is employed as a substrate film for the preparation of the composite film, the poor adhesiveness at high temperatures of the known adhesives still does not allow the preparation of the desired composite film having high heat resistance.
In copending U.S. Patent Application Ser. No. 7/624,043, there is disclosed an extruded multi-layer polyimide film which comprises:
at least one thin layer with a thickness of not more than 10 .mu.m comprising an aromatic polyimide derived from an aromatic tetracarboxylic acid compound and an aromatic diamine having two or more benzene rings; and
a base layer comprising an aromatic polyimide derived from a biphenyltetracarboxylic acid compound and a phenylene diamine,
the at least one thin layer being unitarily arranged on at least one side of the base layer.